Generally, stents are medical devices used to is enlarge lumina of internal organs or blood vessels narrowed by, for example, disease, injury, or surgical operations. Such stents are normally cylindrically shafted and are broadly divided into the following two types: 1) stents having a predetermined amount of elasticity such that they can contract when external force is applied and self-expand when the external force is removed, and 2) stents made of plastic material such that after they are expanded from contracted states, maintain their expanded states.
With regard to the insertion of the above stents in lumina, a widely-used stent insertion device is utilized to allow for easy positioning of the stent. The explanation of this procedure will be omitted herein as this process is well known to those skilled in the art.
U.S. Pat. No. 5,330,500 discloses a stent which, as shown in FIG. 8, comprises a plurality of cylindrical zig-zag elastic units 12, which contract when external force is applied and reexpand when the external force is removed, and a plurality of connecters 13 for connecting the zig-zag elastic units 12 to maintain the same in a cylindrical shape.
Although such a stent utilizing the above zig-zag units 12 attached by the connectors 13 remains in a constant and forceful expanded state, the stent is not flexible nor is it very effective when used to expand lumina which have collapsed. And when used in lumina which are curved in shape, the stent cannot be gently curved, resulting in the zig-zag units 12 and connectors 13 pressing too hard on inside walls of lumina such that inflammation and other complications occur.
Referring to FIG. 9, there is shown a schematic view of another prior art stent positioned in a curved lumen. As shown in the drawing, this stent includes a plurality of zig-zag units 101, a plurality of thread connectors 102 which connect the zig-zag units 101, and a cylindrical cover member 103 made of polyethylene material and which covers the zig-zag units 101 and the connectors 102.
However, the above stent has the drawback of blocking the passageway when used on curved lumina. That is, because the zig-zag units 101 are connected using the thread connectors 102 without any space therebetween and both the zig-zag units 101 and connectors 102 are covered with the cover member 103, when the stent is disposed in a curved lumen, the stent does not gently curve to correspond to a curvature of the lumen, but folds or creases as shown in the drawing so that the passage of the stent, and, thus, the lumen is blocked.
In addition, all prior art stents have the drawback of not having means to prevent the reverse flow of foodstuffs and fluids. Although the human body has natural mechanisms to inhibit the reverse flow of foodstuffs and fluids in the area, for example, where the stomach and esophagus meet, when using the prior art stent in this location it is possible that the esophagus will become damaged because of the reverse flow of acidic foodstuffs and liquids. Further, it is possible that reversed fluid will enter the lungs, leading to lung disease. It is, therefore, not viable to utilize the conventional stent in areas where foodstuffs and liquids need to be prevented from flowing in a reverse direction.
In addition, in the prior stents, the zig-zag units are welded such that each zig-zag unit comes to be formed in a single, integrally formed piece having a plurality of straight sections having a plurality of bends. During the welding process, it is common to use lead material. The lead material, however, can become oxidized within the human body resulting in heavy metals infecting the human body.